View Full Version here: : Solved: Max exit pupil

22-04-2007, 10:58 PM
Without wishing to generate a highly technical discussion, what's the approximate maximum exit pupil for reflectors? As I understand it, there gets a point where the central obstruction becomes 'visible' and therefore the exit pupil is too wide.

Specifically, I'm interested in how this applies to my 8" f/6 Dob or a 10" f/5 Dob (which I may own in the future).

I'm wondering about something like a Vixen 42mm LVW, which on the 8" would give an exit pupil of 7mm (and a field of 2.5 degrees) but on the 10" f/5 would give an exit pupil of 8.4mm. Would this be too much?

Answers in plain English please!



23-04-2007, 09:10 AM
I think there is a simple formula for this, that i have long forgotten. But I'm
sure someone here will know. Like I said I forgot the formula but I think I
remember it went something like....

focal ratio of scope <divided by> focal length of eyepiece

which works for your 42mm and 8" scope above.

Would 8.4mm be too much? well that probably depends on how large your
pupil can dialate, if it could even dialate to that size.

excuse the typo's : )


23-04-2007, 07:03 PM
Hi Rob, that's not quite what I was asking. The human eye can generally dilate to around 7mm, although this decreases with age. So, one school of thought is that an exit pupil that is larger than your own dilated pupil means you are 'wasting' light.

The question I am asking relates to large exit pupils in reflectors, i.e. scopes with a central obstruction. I have read that if the exit pupil gets too large, then you start to see the secondary mirror, which obviously degrades/ruins the image of the object you're trying to look at. This isn't an issue in refractors as there is no central obstruction.

I am 37 years old, so my pupil may not be able to dilate tp 7mm any more, but the lower the magnification the brighter the object will appear. More importantly (for this discussion) the wider the field of view will be.

Therefore, the real question is how wide a field can I obtain with either an 8" f/6 Dob or a 10" f/5 Dob, subject to the limitation of exit pupil?

To give the question more context, this thought process was sparked off by the fact that I couldn't fit the whole Eta Carina Nebula in the fov of my 8" Dob with my widest eyepiece, a GSO 32mm wide view.

Hope some experts out there can provide an answer!



23-04-2007, 08:09 PM
Stopped being so lazy and googled it!

Maximum recommended exit pupil for reflector is around 7mm. Any larger and you might see the shadow of the secondary mirror in the centre of the image.

No such issues for refractors.

Does throw up an interesting issue. With my current f/6 scope, I can go to eyepiece focal lengths of around 42mm. However, if I move to an f/5 scope in the future (which is likely) then the maximum eyepiece focal length would be around 35mm. No point in buying a 41mm Panoptic, then!


24-04-2007, 08:16 AM
I have also thought about this issue. Even though some of the light is "wasted" once the magnification gets too low, the consideration may be more a field of view question, as in your case. Unless the viewing site is very dark, even a younger eye may only dilate to 5 or 6 mm due to ambient light (street lights, the neighbours, local ovals etc). However, if you just want to fit the maximum in the field you might be able to push the magnification a bit lower (especially if it is a refractor), but if gets too low you may also start to get problems with vignetting of the field. So even with a refractor there will be some absolute minimum magnification where all you would see is smaller circle with the same image in it.

My 30mm gives a mag of about 33x and although extended objects should appear "brighter" the sky background also gets brighter so you don't actually see as much as you might think. The stars, being point sources, will appear much the same brightness but the contrast with the background will reduce.

At this mag I get a FOV of around 2 degrees (4 Moons).

24-04-2007, 12:42 PM
I agree. The reason I sold my 35mm Panoptic in the UK was that I only had a short-focal-length refractor at the time and the sky was too bright to make use of it. However, living in Oz I will definitely get out to dark skies where the sky brightness should be much less of an issue, and of course I have my trusty UHC filter.

Having said all of this, the view through my 32mm GSO eyepiece is pretty nice - nice enough that I'm not going to spend more than $500 on a new Panoptic!


24-04-2007, 04:30 PM
The size of a persons maximum pupil dilation (useable exit pupil) depends on a few things. Most important is age. A younger person <30 will have a greater pupil dilation than a person > 30 but < 50 and similarly that person would have a greater pupil dilation than someone >50. Other physical factors will also affect a person's maximum pupil dilation including smoker/non smoker, heavy drinker/non drinker, high blood pressure/low blood pressure etc etc. The greater a person's maximum pupil dilation, the greater the eyepiece exit pupil that will offer them comfortable pleasing views, with good contrast and importantly no secondary shadow in a newtonian, cassegrain, SCT or MCT. The other thing which affects the exit pupil parameters are the darkness of the skies. You can easily use a much larger exit pupil under dark skies than you can under marginal or heavily light polluted skies.

One thing will never change, the smaller the exit pupil the greater the contrast and the lower the aberrations introduced by a persons own eye, eg astigmatism etc. Hence, you are always better to err on the side of being conservative going with a smaller exit pupil, as opposed to a larger one. As a general rule of thumb, people in the 30 to 50 age bracket should not exceed an exit pupil of about 7mm under dark skies and about 6mm under any form of light polluted skies. I am 48 and use a 6.88mm exit pupil under dark skies with good views. Under less than dark skies the view starts to lose a little contrast and I would not like an exit pupil any larger. A person under 30 yrs of age could "possibly" go up to 1mm over those parameters. If you limit yourself to a maximum exit pupil between 6 and 7mm you should be very safe.

Clear Skies
John Bambury

14-05-2007, 09:32 PM
Hi Morton,
I think I understand what you were trying to know.......
Check out this article by Nagler talking about this subject;

To explain:
The size of the shadow cast by the secondary into the beam of light emerging from the eyepiece will be the result of its size in proportion to the primary.

So using your example of the 10" F5 with say a 42mm eyepiece.
Exit pupil is, as you say; 8.4mm.
If the diagonal was a 54mm, then this represents approx 21% of the primary on diameter.
21% of 8.4mm = ~1.76mm size shadow
If we assume your pupil size at ~6mm then the shadow is about 29% (on diameter) of the beam of light entering your eye.

Taking an extreme example to make the point:
Make the eyepiece a 60mm in your 10"F5 Dob.
Exit pupil = 12mm
21% of 12mm= ~2.5mm shadow in the centre of the light beam.
About 42% of the beam going into your eye.

I suspect from the above that you could easily go to 50mm on the 10" F5 without the shadow being that noticeble. Different story during daytime where you would pick up the shadow quite clearly.

A bigger problem with the very low powers will be the sky "whiting out" on you! The magnification is so low that the scope picks up all the backlight in the sky and you lose contrast.

Many will say that going above 7mm wastes aperture because the light is falling outside your pupil but ignores the fact that you get the wide field of view you were looking for......... a problem on the bigger Dobs.

:-) Clear as mud?

Greg Reilly

15-05-2007, 11:04 PM
It all makes sense, kinda...

Since I've just bought a second-hand 35mm Panoptic (it's in the post) the argument is now moot, cos my eyepiece-buying days are now over!